US President Barack Obama on Sunday demanded the United Nations and China ramp up pressure on North Korea for its "belligerent behavior" and warned against turning a blind eye to its deadly acts.

US President Barack Obama on Sunday demanded the United Nations and China ramp up pressure on North Korea for its "belligerent behavior" and warned against turning a blind eye to its deadly acts.

Using the global stage of a G20 summit in Toronto, Obama said the international community had to act and stand by South Korean President Lee Myung-Bak.

The 15-nation UN Security Council must ensure there is "a crystal clear acknowledgement that North Korea engaged in belligerent behavior that is unacceptable to the international community," Obama insisted.

He was speaking just a day after G8 leaders issued a strong condemnation of the alleged North Korean sinking of a South Korean warship near the disputed Yellow Sea border in March.

A multinational investigation earlier this month found the isolated Stalinist state had torpedoed the 1,200-tonne Cheonan, leading to the deaths of 46 sailors.

But North Korea has angrily rejected the charges, instead accusing the United States of being behind the incident and warning of military reprisals if it is slapped with any sanctions.

The United States and Seoul have led a push for a UN censure of Pyongyang for the sinking of warship, but the Security Council has yet to issue a formal condemnation.

Obama said he believed South Korean President Lee "has shown extraordinary restraint" and said the world needed to ensure that Pyongyang comes back to six-party talks on its suspect nuclear program.

The US leader met with Lee on the sidelines of talks of the Group of 20 developed and emerging economies in the eastern Canadian city of Toronto, and vowed Washington stood "foursquare" behind him.

"It is absolutely critical that the international community rally behind him and send a clear message to North Korea that this kind of behavior is unacceptable," he added.

He also held talks with Chinese President Hu Jintao, and said he had been "blunt" with him on the issue of North Korea.

While he understood that North Korea and China were neighbors, Obama said: "I think there's a difference between restraint and willful blindness to consistent problems.

"My hope is that President Hu will recognize as well that this is an example of Pyongyang going over the line."

Beijing is a close ally of Pyongyang, providing the impoverished nation with an economic lifeline, and has been reluctant to endorse a UN condemnation over the ship sinking until it has assessed the evidence for itself.

G8 leaders on Saturday condemned Pyongyang for the alleged North Korean attack that sank the South Korean warship.

Recalling the findings of the a multinational investigation, the statement said: "We condemn, in this context, the attack which led to the sinking of the Cheonan."

Japanese Prime Minister Naoto Kan also said Sunday he had asked Hu to condemn North Korea's alleged sinking of the warship.

Citing the G8 declaration, Kan said he "encouraged China to move in a similar direction," pointing out Beijing's condemnation was "necessary."

CIA chief Leon Panetta meanwhile warned the world faces a "dangerous period" as North Korea's ailing leader Kim Jong-Il tries to cement his son as his successor.

Barring a dramatic change of course, Kim's son Jong-Un seems set to inherit the reins of the hermit nation, which remains locked in confrontation with the West over its nuclear ambitions.

"Part of the skirmishes that are going on are in part related to trying to establish credibility for the son," Panetta said. "And that makes it a dangerous period."

North Korea said on Sunday it was open to inter-Korean military talks to address the sinking of the South Korean warship, and called on the United States to cease its involvement in the case.

The North, however, renewed a demand that the South first allow Pyongyang to carry out its own inspection to verify the facts of the case -- a condition Seoul has refused.